There exists a need for simple diagnostic tests for common diseases which can be carried out by untrained personnel. Such tests facilitate home or doctor""s office testing as opposed to more complicated procedures which require that the analysis be carried out in an outside reference laboratory. A common format for these tests involves the use of an immunostrip. Typically, this format involves a matrix of a material through which a fluid test sample, such as urine, can flow by capillarity. The matrix, typically in the form of a strip through which the test fluid flows horizontally, contains an antibody specific for the analyte whose presence and/or concentration is being sought which antibody bears a detectable label, so that the presence and/or concentration of the analyte in the test fluid can be determined by detection of the signal emitted from the detectable label. A classical embodiment of such a device, sometimes referred to as an immunochromatographic strip, is illustrated by FIG. 1. Referring to FIG. 1, strip 10 bears a labeled antibody, specific for the analyte under investigation, in zone 13 which binds with the analyte in the fluid test sample applied to the wicking zone 12 of the strip 10 which may contain a buffer and flows along the strip to form an immunocomplex which further migrates due to capillary action through the capture zone of the strip 14 and the optional detection zone 16. In the capture zone 14 there is immobilized the analyte or a derivative thereof which is immunoreactive with the antibody and is able to capture labeled antibody which has not reacted with analyte in the fluid test sample. The signal from the labeled antibody captured in the capture zone is measured and related to the concentration of analyte in the test fluid in an inverse relationship since the greater the concentration of analyte in the test sample, the amount of labeled antibody which is unbound and thereby free to specifically bind with the analyte immobilized in the capture zone is diminished. Detection zone 16 is optional but can contain immobilized anti-mouse IgG to bind the analyte/labeled binding partner complex and thereby serve as a means for verifying that the test has been carried out correctly.
Urine is a common test fluid for this sort of assay, and it has now been discovered that urea, whose concentration in urine will vary tends to interfere with the binding between the antibody and analyte in the system. The degree of such interference will vary depending on the particular antibody and antigen involved in the assay. Urea is synthesized in the liver as the final product of amino acid catabolism. Urea excretion is proportionate to protein intake and dependent on the protein metabolism, it increases with a high protein diet. Normal adults excrete about 20.6 g of urea per day. The interferences with the antigen/antibody binding tend to cause the assay to have a negative bias when the urine test sample contains more than the normal amount of urea. It would be desirable, and it is an object of the present invention to provide a method and a device which reduce or eliminate the bias caused by the fluctuations in urea concentration in urine.
The present invention is a method for the analysis of an analyte in a urine test sample in which a labeled antibody specific to the analyte is combined with the urine and the concentration of analyte in the urine is determined by the magnitude of the response from the labeled antibody which is a function of the amount of binding of the labeled antibody with the analyte. According to the present invention, the assay is improved upon by maintaining the concentration of urea in the urine above a threshold value which is the concentration at which increases in urea do not affect the binding between the analyte and the labeled antibody. Also included within the scope of the present invention is a test device for carrying out the improved assay.